Action delete API rework - support for action file deletion

[mahesh-orch]

• Action delete API modification to have option to delete action database entry without removing files from disk
• From CLI st2 action delete <pack>.<action> will delete database entry only
• From CLI st2 action delete --remove-files <pack>.<action> will delete database entry and files.
• If action DELETE method has additional argument as {"remove_files": true} in the body then it will remove related action files from disk along with database entry.
• {"remove_files": false} or no additional argument in action DELETE method body will remove only database entry.

[mahesh-orch]

@m4dcoder I have added unit tests, updated existing unit test and done other minor changes as suggested. Please review. Thanks.

[m4dcoder]

LGTM

[amanda11]

Just a minor comment on wording in the schema, but @nzlosh point more important.

[mahesh-orch]

@nzlosh I have changed CLI action delete optional argument -rf to -r.
@amanda11 I have updated the description of ActionDeleteSchema remove-files argument.
Please review. Thanks.